The present invention is in the field of solid state color cameras using solid state image sensing devices such as charge coupled devices (CCD) and metal oxide semiconductors (MOS).
Solid state cameras have constructions which include solid state image sensing devices, such as CCD's or MOS devices, which are arranged in the form of a lattice on a two-dimensional plane so that images resulting from a camera lens system are discretely sampled. Such solid state color cameras have been suggested as suitable image sensing means for a small sized color television camera apparatus.
However, since this type of solid state camera carries out two-dimensional sampling by means of three primary color picture elements (pixels) arranged to provide images in two dimensions, sampled outputs have spatial frequency spectra having carrier frequency components at predetermined frequency positions with respect to a carrier frequency on a base band on the basis of a horizontal directional frequency and a vertical directional frequency which are determined on the basis of the pitch between individual image sensing devices which constitute each picture element.
If carrier components other than that of the base bands are left intact, they provide a source of difficulty such as a moire or a cross-color phenomenon on reproduced images of color televisions when the images are reproduced on the basis of sampled outputs. Among methods suggested for solving this problem include the use of an optical low-pass filter in which the light received from the camera lens system is optically diffused on the basis of a point diffusion characteristic of an optical filter to form a trap within a spatial spectrum of frequency by the utilization of birefringence of a crystal plate.
The above-described low-pass filter is inserted between a solid state image sensing device and a solid state camera lens system. The solid state camera is arranged, as shown in FIG. 1, such that the solid state image sensing devices such as CCD image sensors are arranged in sequence at the interval having a pitch P.sub.x, for example, of 17 microns in the horizontal direction (the H direction) and CCD image sensors are positioned at another interval of pitch P.sub.y, for example, 13 microns, in the vertical direction (the V direction).
Referring to FIG. 1, in a picture element group constituted by CCD image sensors, there are blue color elements 1B disposed adjacent green color elements 1G in the H direction and green color elements 2G disposed adjacent the green color elements 1G in the V direction. Further, red color elements 2R are positioned adjoining the green color elements 2G in the H direction and adjoining the blue color elements 1B in the V direction. Consequently, each set of four elements 1G, 1B, 2G and 2R is sequentially arrayed in both the H and V directions and thus a two-dimensional solid state image sensing device pattern is formed. When images received from the camera lens system of the solid state camera mentioned above are sampled, the sampled outputs have a spatial frequency spectrum as shown in FIG. 2. The lateral axis and longitudinal axis of FIG. 2 are expressed as numerical values normalized using such values as P.sub.y v/2.pi. and P.sub.x u/2.pi. on the basis of the pitch of the picture elements P.sub.x and P.sub.y in the H direction and V direction, respectively. In addition, the arrow marks R, G, B at each frequency position denote phases of carrier components of the red, green, and blue color signals.
As will be seen from FIG. 2, the sampled outputs derived from the solid state camera constructed as in FIG. 1 have carrier components at frequency positions f.sub.x =0, 1/2, 1, . . . in the H direction and at the frequency positions f.sub.y =0, 1/4, 1/2, 3/4, . . . in the V direction.
The base band component is produced at the frequency position (f.sub.x =0, f.sub.y =0), and the carrier components generated at the other frequency positions are unnecessary signal components which may provide sources of deteriorating picture qualities of images when the sampled outputs are reproduced.
For example, the carrier components produced at the frequency point (f.sub.x =1, f.sub.y =0) as a center will produce a moire on a reproduced picture of an image signal if the object to the picture has a fine stripe pattern comprising black and white stripes extending in the vertical direction.
On the other hand, the carrier components produced with the frequency position (f.sub.x =1/2, f.sub.y =0) as a center will produce the cross-color phenomenon on the green and magenta colors if an object to be pictured has a slightly coarse longitudinal directional stripe.
Furthermore, the carrier components produced at the frequency position (f.sub.x =0, f.sub.y =1/2) as a center will produce a flicker on the vertical edge of the reproduced image. Hence, if filtering means are provided for eliminating the components produced at frequency positions other than the frequency position (f.sub.x =0, f.sub.y =0) without attenuation of the base band component corresponding to the frequency position (f.sub.x =0, f.sub.y =0), the picture of the object can be reproduced with high resolution on the basis of the image signal produced from the solid state color camera.